1. Technical Field of the Invention
The invention relates to the field of frequency measuring, in particular to extremely accurate difference frequency measuring as it is used in the area of measuring instruments. Such accurate frequency measurements are for example required in scanning atomic force microscopes or other scanning microscopes, so-called scanning probe microscopes if their oscillation characteristics are used for measuring. In the area of thermogravimetry they can be used to determine extremely small masses in the ng-range. The invention relates in particular to the principle of a hybrid frequency measuring device or frequency detector, i.e. a frequency measuring device or frequency detector comprising both digital and analog components or groups of components. The invention also relates to a particularly advantageous use of such a frequency detector for evaluating the frequency measurements of a scanning probe microscope. Thus, for example, a frequency measuring arrangement according to the invention can be at least partially integrated in a measuring head of a scanning probe microscope, thus enabling a robust, and at the same time sensitive, measuring arrangement which provides advantages compared to the state of the art.
2. State of the Art
In scanning probe microscopy, often an electronic device is used as a probe which records the interaction between the probe and a subject being measured, i.e. the specimen. To this effect, essentially two methods are being used: either contact measuring where the static influencing of the probe by the specimen is measured, or non-contact measuring where the influencing of the probe by the specimen is recorded as a resonance frequency change in the probe. However, since most of the time these frequency changes are very small in relation to the uninfluenced free resonance frequency of the probe, the requirements of the sensor electronics are high. In this case, the sensor electronics must be in a position to convert the small signal changes of the probe into a voltage or a numeric value which can be further processed. Obviously a host of problems need to be solved in this process. There are not only temperature-dependent factors and problems with long-term stability but also adequate measuring resolution and linearity need to be achieved, and even tolerances of the components must be taken into account.
Such a solution has for example been described by T. Albrecht in the Journal of Applied Physics, Vol. 69, 1991, pp 668ff, in the paper xe2x80x9cFrequency Modulation Detection Using High-Q Cantilevers for Enhanced Microscopy Sensitivityxe2x80x9d. The method described there attempts to determine the resonance frequency change of the sensor by means of a known filter characteristic, usually a band pass or low pass. In so doing, the frequency dependence of the transfer characteristic of the filter used is used to obtain a measurement value via the amplitude change of the filter output signal when changing the frequency of the filter input signal. While with this method or with such sensor electronics it is possible to achieve a measuring resolution which is sufficiently high, the linear relationship between the frequency change of the probe and the output value of the sensor electronics is however only present in a narrow band and is only approximate. Furthermore, the temperature independence of such an arrangement is not always satisfactory; it reacts strongly to component tolerances. In addition, such sensor electronics can be adapted only with difficulties to various sensors with widely differing resonance frequencies and/or resonance frequency shifts because usually several inductivities and/or capacities matched to each other need to be changed.
Another attempt at solving the task described comprises frequency measurement by means of an analog phase-locked-loop (PLL) component. Such an attempt has for example been described by Roland E. Best in xe2x80x9cPhase-Locked Loops, Theory, Design and Applicationsxe2x80x9d, 1st ed., McGraw-Hill, New York, 1984, Chapter 2. There, a frequency signal emanating from a transmitter is measured by means of an analog PLL. The already processed transmitter oscillation is used as an input signal of the PLL phase detector while the VCO control signal, i.e. the voltage controlling the oscillator of the PLL serves as an output value of the electronics and is used as a measure for the frequency change of the transmitter. Such an arrangement provides quite good linearity and good noise suppression and the component tolerances are not too critical. However, good measuring frequency is only achieved within a limited frequency measuring range which means that the range of applications is limited. Furthermore, temperature stability and long-term stability of such an arrangement can only be achieved at considerable expense as is the case with most analog circuits. Moreover, such an arrangement is difficult to adapt to different sensors with strongly differing resonance frequencies and/or different resonance frequency shifts.
A third attempt at a solution is provided by C. Loppacher et al in xe2x80x9cFast Digital Electronics for Application in Dynamic Force Microscope Using High-Q Cantileversxe2x80x9d, Applied Physics A, pp 66-215, 1998. This provides a description as to how the frequency signal and/or the phase shift of a sensor can be measured by means of a purely digital PLL. This design solves most of the problems of the above-mentioned two methods. It is insensitive to temperature, it is stable in the long term, linear, easily adaptable to various sensors and almost independent of component tolerances. However, it requires considerable expenditure in components space and electrical energy which not only results in high cost but also renders such a solution unusable for applications which demand small size and/or low energy consumption. Of course the high costs mentioned also have a prohibitive effect.
A further method is described in the patent application WO 96/24026, xe2x80x9cTapping AFM with Phase or Frequency Detectionxe2x80x9d. This attempt shows how distance measuring to the specimen can be achieved by means of an oscillating probe, with the dependence of the amplitude of the probe oscillation from the distance to the specimen being exploited. By means of amplitude measuring it is then possible to derive the distance to the specimen. One problem of this method is due to the fact that in the case of probes of high quality Q, the amplitude changes only very slowly and that consequently measuring is also very slow. This renders the described method unusable for all measurements where speed is important, i.e. unusable for the majority of applications. Moreover the relationship between the probe amplitude and the resonance frequency is extremely complicated which renders the interpretation of the measured data considerably more difficult.
The aforementioned thus leads to the object of this invention which is to disclose a method and a device, i.e. a circuit, which as far as possible achieves all the advantages of the latter fully-digital attempt at considerably reduced technical and financial expenditure. Adequate independence of temperature fluctuations and component tolerances are to be achieved, as are good long-term stability, the necessary linearity and measuring resolution. In other words, a solution is to be found which provides practically the same advantages as the last-described method, but without the problems of energy, space and cost.
In a nutshell, the invention is a frequency detector which in a useful and thus inventive way combines analog and digital design so as to achieve optimal effect with minimized technical and financial expenditure. In principle the detector works in an analog way in the range where high reaction speeds and/or complicated mathematical calculations are required, while it works digitally in the range where high accuracy, in particular frequency accuracy and/or stability are required, with the detector comprising the respective components.
It is a particular advantage that some of the necessary components, in particular analog components, are readily available in the market and are therefore available in good quality at economical prices. At the same time the entire digital part can for example be implemented by means of a microprocessor and by respective software, thus providing almost unlimited flexibility.
While hybrid circuits are known in principle, e.g from patent application EP 661 815, xe2x80x9cFrequency Synthesizerxe2x80x9d, such a circuit can only generate one frequency and can therefore not fulfil the above-mentioned task, i.e. for example carrying out highly precise and fast measurement of an input frequency relative to a reference frequency.
In a preferred embodiment of the invention which is subsequently mentioned, there is a further advantage from a particularly advantageous arrangement of the A/D transition. It can be designed so that one and the same frequency detector practically at the same time can provide both an analog and a digital output signal which in turn ensures a flexibility which could not be achieved with any of the previous solutions.
Furthermore, the invention can be very simply adapted and can be simply modified for the use of various sensors, e.g. apart from self-oscillating sensors, extraneously driven sensors, with the phase-correct and frequency-correct drive being able to be delivered by a circuit according to the invention. An embodiment for this modification is also shown below.
In summary, and without making any claims for being exhaustive, the invention can be described as an electronic frequency measuring device which is particularly suitable for precision measuring of small frequency differences and which features an advantageous hybrid design, with the device essentially comprising three groups of components:
1. An analog input group which is supplied with an input frequency to be measured and an analog feedback signal, said input group forming an analog difference signal therefrom, which signal depicts the difference between the input frequency and a given reference frequency;
2. A digital mixed group which forms a sum frequency from the difference signal and the reference frequency;
3. An analog output group which forms the feedback signal from the digital sum frequency.
In particular the option of outputting a synchronised frequency control signal, e.g. for driving a non-self-oscillating sensor, is provided as an expansion.
Overall, the solution presented here essentially provides the same advantages as does the last-mentioned method of the state of the art, except that this solution also elegantly solves the there presented problems of energy, space, and cost.
Further advantageous embodiments result from the following description of the embodiments.